Use interesting UTF-8 characters in the "clean" test

Attempting to confirm JENKINS-20410 and JENKINS-22434.
Add more interesting characters, correct encoding mistake made earlier
Specify file content encoding explicitly in GitAPITestCase
Add a composed Unicode character to clean test
More attempts to assure unicode file names are properly cleaned
Another test of [JENKINS-22434], shows failure if LANG=C
Use --progress only when it's supported

This option is relatively new in 1.7.1.
See
http://stackoverflow.com/questions/21735978/jenkins-git-fetch-fails-with-unknown-option-progress
for the detective work
Warn if git version does not support credentials

[JENKINS-21887] The git credentials support was not added until git 1.7.9.

This commit will only provide a warning in the log file, but will
still attempt to execute the credentials operations.
log executed command on build console to help user diagnose issues
[Fix JENKINS-22039] Use new temp dir for remote polling credentials check

The default behavior was to create a new git directory in the system
temporary directory.  Since git repositories apply to all child
directories, that had the bad result of making the system temporary
directory appear as though it were under git control.

That also caused spurious automated test failures in the git-client
unit tests.
[maven-release-plugin] prepare release git-client-1.8.1
[maven-release-plugin] prepare for next development iteration
Update README.md

Simplify phrasing in the README
Add tests of Revision object
Cover a few more of the Revision branches / statements
Sanitize whitespace in RevisionTest
Add test of GitException with no message string
Add test of GitLockFailedException
Add test of legacy Tag object
Add test of legaxy IndexEntry class